The conventional alternative energy is developed for replacement of petro energy and the wind power is chosen to generate electric power. However, the speed of wind cannot be controlled precisely, if the wind velocity is too low, the turbine cannot be started, if the wind is too high, the turbine is easily damaged. The efficiency of generating electric power varies according to different wind velocities.
The inclination angle, the length and the surface area affect the speed of the blades. Under the same wind velocity, the larger the inclination angle of the blades, the torque and the air resistance become higher, and vice versa. Therefore, the adjustment of the inclination angle, the length and the surface area of the blades according the change of the wind velocity is necessary, wherein the adjustment of the inclination angle is easily achieved. FIG. 1 shows a conventional wind turbine and has an exterior wind velocity sensing device to detect the wind velocity. The wind turbine comprises a base 10 with a housing 20 connected thereto, and multiple blades 201 are provided. A receiver 30 is located in the housing 20 and operated corresponding to the wind velocity sensing device so as to calculate the inclination angle for the blade 201 at different wind velocities. The receiver 30 is electrically connected to a motor 301 which adjusts the inclination angle of the blade 201 by a transmission unit 302. The housing 20 is connected to a speed-increasing gear set and a generator 50 via a transmission shaft to increase the efficiency of the turbine. However, the exterior wind velocity sensing device and the motor 301 require electric power to monitor the wind velocity and adjust the inclination angle of the blade.
Besides, when the wind is too high, the torque generated by the blade 201 increases so that the speed of the blade increases. The significant centrifugal force may break the blade and the generator 50 may be burned so that the speed needs to be controlled. An electric speed-reducing device 60 is located between the base 10 and the housing 20 so as to reduce the speed of the blades 201. Nevertheless, it requires a lot of power to reduce the speed of the blades 201 when the wind velocity is high, and the electric speed-reducing device 60 will be burned after a long period of operation.
FIG. 2 and FIG. 3 disclose another conventional wind turbine 70 which has a tail wing unit 701 which swings to change the direction of the turbine 70 relative to the wind so that the wind force applied to the blades is reduced and the speed of the blades is reduced. However, the swinging turbine 70 obviously is not stable.
For the stationary blades, as shown in FIG. 4, assume that the wind velocity is Vwind and an attack angle α is formed between the blade 80 and the Vwind. Assume that the length of the blade 80 is R, the revolution of the blade 80 is N, and the inclination angle β at the outside of the distal end of the blade 80 is almost 90 degrees. When the N is zero, the Vwind is perpendicular to the α and β of the blade 80 so that the blade 80 cannot generate push force. When the N reaches a large number and the a is a positive value, because the blades 80 are stationary, the β is remained to be almost 90 degrees. As shown in FIG. 5, the relative velocity of the Vwind to the blade 80 is represented by V, and the tangential velocity that the Vwind generates relative to the blade 80 is represented by Vt, the relationship between the Vwind and the Vt is expressed by the equation Vt=(2πNR)/60. The Vwind generates a lift coefficient CL, the relationship between the CL and the α is expressed in FIG. 6. When the α is larger than a pre-set value, the CL will be quickly reduced to make the N of the blade 80 stall.
For the conventional wind turbines, the manufacturing and maintenance expenses for the wind velocity sensing device, the motor and the electric speed-reducing device are high, and the blades cannot be stopped when the wind velocity is too high. Besides, when the wind velocity is high, the speed of the blades cannot be maintained at a constant speed so that the electric speed-reducing device and the blades are easily damaged. Besides, the wind turbine with the tail wing makes the wind turbine swing all the time and therefore causes fatigue and instability. These drawbacks are needed to be improved.
The present invention intends to provide a wind turbine that improves the drawbacks mentioned above.